The invention relates to the field of heat-sealable cartons and, more particularly, to a process for forming a heat-sealable carton, and to the carton itself, which can be rapidly sealed and which exhibits good seal integrity at high temperatures.
Many varieties of cartons or containers formed from folded paperboard are known in the packaging art. For example, in packaging food items, such as frozen foods, it has been the practice to form a carton from plastic coated paperboard and to seal the carton closure flaps after insertion of the food items by automatic machinery, such as shown in the patent to Gobalet (U.S. Pat. No. 2,984,598), owned by the assignee of the present invention. Gobalet coats a water-based adhesive, such as polyvinyl acetate emulsion, on one of the carton flaps and then dries the water-based adhesive with heating lamps. The dried adhesive exhibits a high degree of contact tackiness. After the adhesive is dried, the adhesive coated surface of the flap is pressed against the carton and cooled to form a seal. The carton bonding method of Gobalet allows rapid bonding of the carton flaps and exhibits good sealing characteristics at the temperatures normally encountered by such packages, i.e., between approximately 0.degree. F. and 100.degree. F.
Recently, there has been a trend within the food industry to produce packaged foods which can be placed directly in micro-wave or conventional ovens for cooking or reheating. Many such packaged foods are cooked while covered or sealed, so as to retain moisture, flavor, and to prevent splatter in the oven. While heating of the package within a microwave oven rarely present problems, the temperatues necessary for cooking or reheating of packaged foods in a conventional oven often exceed 217.degree. C.(425.degree. F.). Many such so-called "ovenable" packages are coated with a heat-resistant polyester resin coating rather than a polyethylene coating or the like which would melt at oven temperatures. The polyester resin coating also prevents the paperboard from absorbing grease and cooking juices during cooking.
Polyester resin is a thermosetting resin which can be used as an adhesive when heated above approximately 225.degree. to 550.degree. F. with pressure being applied to the surfaces being bonded. An adhesive seal made with polyester resin exhibits good seal integrity at elevated temperatures (i.e., up to 230.degree. C. or 450.degree. F.). However, the compression needed to set such adhesives has previously been a substantial hindrance, since the possibilities for firm compression is often severely limited. In addition, thermosetting resins exhibit good bond strength only after the bond has cooled, which means that some method must be used to hold the carton edges together while the bond is curing. When using automatic carton sealing machinery, the long curing times of thermosetting resins also reduces the number of cartons which can be sealed in a given period of time and increases the size of the machine.
It is therefore an object of the present invention to provide a method of forming a heat-sealable carton exhibiting good seal integrity at high temperatures.
It is another object to provide a method of forming a heat-sealable carton which can be performed rapidly on a conventional carton sealing apparatus with little modification to the apparatus.
It is a further object to provide a heat-sealable carton, useful as a cooking or heating container for foods, which is formed by the process of the present invention.